1. Field of the Invention
The invention relates to a carriage driving apparatus for reciprocating a carriage by a timing belt wound around a drive pulley and a driven pulley. The invention also relates to an image reading apparatus and an image recording apparatus including such a carriage driving apparatus.
2. Description of Related Art
As an image reading apparatus in a copy machine and a multifunction machine, there is known a flatbed scanner having a reading table in which is disposed a carriage on which an image sensor such as CCD (Charge Coupled Device) or CIS (Contact Image Sensor) is mounted in order to scan or read an image on a document placed on the reading table. As a carriage driving apparatus for reciprocating the carriage in such a flatbed scanner, there is known an apparatus where a timing belt is wound around a drive pulley mounted on an output shaft of a drive motor, and a driven pulley, and the carriage is coupled with the timing belt in order that rotation of the drive pulley is transmitted to the carriage via the timing belt, so that the carriage is slid on and along a guide shaft or the like. On the other hand, in an image recording apparatus, e.g., serial printer, such a carriage driving apparatus is used to reciprocate a carriage on which a recording head is mounted.
JP-A-2000-37921 discloses a conventional arrangement for supporting a driven pulley in a carriage driving apparatus, where a timing belt coupled with a carriage is wound around a drive pulley attached on an output shaft of a drive motor, and the driven pulley as attached to a bracket or a pulley holder of a machine incorporating the carriage driving apparatus. The pulley holder is displaceable along a guiding slot formed in a chassis frame, or a base, of the machine. In order to give a tensile force to the timing belt, the pulley holder is biased by a biaser in the form of a compression spring interposed between the pulley holder and a vertical part formed by cutting the base and bending the cut portion upward to stand. In the vicinity of the pulley holder, a stopper or a restrictor is screwed to the base in order to limit sliding movement of the pulley holder toward the drive pulley, that is, in a direction to slacken the timing belt or to the “slackening side”. Thus, the timing belt is kept tense without slack, thereby ensuring the transmission of rotation of the drive pulley to the carriage via the timing belt.
As described above, the restrictor is screwed to the base in order to limit the sliding movement of the pulley holder, thereby preventing the timing belt from slacking to such a degree that the timing belt disengages from the driven pulley. However, this arrangement requires to screw the restrictor at a suitable position after the attachment of the pulley holder at the guiding slot, thereby requiring components such as the screw and the restrictor, as well as steps of assembling these components. The increase in the number of components and assembly steps pushes up the cost of the carriage driving apparatus.
In a case where components including the restrictor and the screw securing the restrictor are omitted, no problem occurs with the timing belt when the pulley holder is slid along the guiding slot in the direction to slacken the timing belt during a normal reading operation in which an image is read while the carriage is being reciprocated. However, when an impact is imposed on the carriage driving apparatus to displace the carriage by a large amount, during transportation of the machine or in other situations, the pulley holder may greatly slide in the direction to slacken the timing belt against the biasing force of the compression spring, resulting in disengagement of the timing belt from the driven pulley, or disengagement of the pulley holder from the guiding slot.
To prevent such an undesirable great movement of the carriage during transportation of the machine or in other situations, it has been proposed to dispose a retainer which temporarily fixes the carriage to the base of the machine. Temporarily fixing the carriage by means of the retainer is much meritorious particularly when the carriage driving apparatus is used in an image reading apparatus such as flatbed scanner that uses a CCD as the image sensor, since a weight of the carriage is increased by mounting on the carriage the CCD, which includes a reflecting mirror and others. However, employment of the CCD in the image reading apparatus is undesirable since the cost and size of the image reading apparatus increase.
A CIS that is smaller in size, more lightweight, and lower in cost than a CCD is suitably employed in an image reading apparatus when the size and cost of the image reading apparatus are desired to be small and low. A carriage on which a CIS is mounted is lightweight as compared to a carriage on which a CCD is mounted. However, when the carriage driving apparatus includes neither the restrictor that limits sliding movement of the pulley holder, nor the retainer that temporarily fixes the carriage to the base of the machine to prevent displacement of the carriage, the timing belt or the pulley holder may disengage during transportation of the machine or in other situations, as described above. When the retainer is employed for temporarily fixing the carriage to the base, the number of components and assembly steps increase, thereby pushing up the cost of the image reading apparatus, similarly to the case of the CCD.